The invention relates to apparatuses for locating transmitters, and in particular to methods of locating terrestrial transmitters by satellite.
Document U.S. Pat. No. 3,949,400 discloses the use of a geostationary satellite and a ground station for determining the position of a beacon in a predetermined zone. The beacon transmits a particular signal enabling it to be identified. The satellite has an antenna beam scanning the intended zone in a predetermined cycle. The antenna beam detects signals only in a very precise direction. When the antenna beam is pointed towards the beacon, then the satellite intercepts the signal from the beacon and thus determines the position of the beacon as a function of the position of the satellite and the position of the antenna beam in its scanning cycle. It returns the position of the beacon to a ground station.
Document JP-08 263 853 discloses using a control unit for steering an antenna towards a transmitting communications satellite. The control unit calculates the amplitudes of the movements to be imparted to the antenna so as to cause it to face the intended satellite and it controls antenna movement. More precisely, the antenna is steered by moving the antenna so as to determine the position in which the signal it receives from the transmitting satellite is at a maximum.
Nevertheless, those apparatuses and their methods of operation are suitable only for locating transmitters transmitting signals that are specifically intended to be detected.
Solution and performance analysis of geolocation by TDOA.IEEE Transactions on Aerospace and Electric Systems, Vol. 29, No. 4, October 1993, also discloses the use of a plurality of instruments in separate locations that are sufficiently spaced apart to enable travel time differences and/or Doppler differences to be measured.
The document xe2x80x9cPrincipes de traitement des signaux radar et sonarxe2x80x9d [The principles of processing radar and sonar signals], by Le Chevalier, published by Editions Masson, and the document xe2x80x9cMxc3xa9thodes à haute rxc3xa9solutionxe2x80x9d [High resolution methods] by S. Marcos, published by Editions Hermxc3xa8s, describe methods relating to locating transmitters by antenna systems. That type of device constitutes the basis for implementing processing by amplitude radio location, by phase radio location, or by the xe2x80x9chigh resolutionxe2x80x9d method enabling transmitters to be located on the basis of measurements performed on antenna arrays.
The document xe2x80x9cPrincipes de traitement des signaux radar et sonarxe2x80x9d, by Le Chevalier, published by Editions Masson, also describes a method of locating a transmitter by using the properties of a directional antenna. The emitter is located by determining the pointing direction of the antenna for which the energy received by said antenna is at a maximum.
Such apparatuses do not enable non-authorized or jamming antennas to be detected which might disturb the operation of telecommunications systems. Those apparatuses require either a plurality of spaced-apart satellites each provided with an antenna for locating a transmitter, or else a single satellite presenting a plurality of antennas.
There therefore exists a need for an antenna which resolves one or more of those drawbacks. In this context, the invention provides a method of locating a terrestrial transmitter by means of a geostationary satellite provided with an antenna, the method comprising the steps consisting in: measuring, at the satellite, and at at least two different instants, three orthogonal components of an electromagnetic field transmitted by the terrestrial transmitter; calculating at least two electromagnetic field vectors from the measurements; determining the propagation direction of the electromagnetic field from the determined vectors; and locating the transmitter at the point of intersection between the surface of the earth and a straight line colinear with the propagation direction and passing through the satellite.
In a variant, the propagation direction of the electromagnetic field is determined by identifying a vector for which the square of the sum of its scalar products with the determined electromagnetic field vectors is at a minimum.
In another variant, the propagation direction of the electromagnetic field is determined by determining a vector defined by the sum of the vector products between at least two electromagnetic field vectors.
In another variant, the calculation step comprises multiplying the measured field components by the inverse of the antenna transfer function.
In yet another variant, locating is performed by at least two satellites, the method further comprising a step of triangulation on the locations determined by the two satellites.
Another implementation of the invention provides a method of using a moving satellite to locate a terrestrial transmitter transmitting a linearly polarized electromagnetic field, the method comprising the steps consisting in: measuring, at the satellite, and at at least two different instants, three orthogonal components of an electromagnetic field transmitted by the terrestrial transmitter; calculating electromagnetic field vectors from the measurements; for each measurement, determining a plane containing the propagation direction of the measured electromagnetic field and the satellite, on the basis of the calculated vectors; and locating the transmitter at the intersection of a plurality of circles where the planes intersect the surface of the earth.
Another implementation of the invention provides a method of using a plurality of satellites to locate a terrestrial transmitter transmitting a linearly polarized electromagnetic field, the method comprising the steps consisting in: measuring, at the satellites, and at a common instant, three orthogonal components of an electromagnetic field transmitted by the terrestrial transmitter; calculating electromagnetic field vectors from the measurements; for each measurement, determining a plane containing the propagation direction of the measured electromagnetic field and the satellite, on the basis of the calculated vectors; and locating the transmitter at the intersection of a plurality of circles formed by the intersections of the planes and the surface of the earth.
In a variant, these methods further comprise an additional measurement step using differential Doppler measurements, interferometry, or radio direction finding.
In yet another variant, these methods further comprise a step of frequency filtering the measured electromagnetic field.
The invention also provides a transmitter-locating satellite having an antenna and means for implementing the locating methods of the invention.